1. Field of the Invention
This invention relates to the production of flat or curved sheets of thermally toughened glass, such as are used for example as motor vehicle side or rear windows.
2. Description of the Prior Art
Such a glass sheet may be toughened by heating the glass sheet to a temperature near to the softening point of the glass and then subjecting the glass sheet to a rapid chilling effected by directing quenching jets, usually of air, simultaneously against both surfaces of the glass sheet. Cooling of the surfaces of the glass sheet in this manner sets up centre-to-surface temperature gradients through the thickness of the glass sheet which are maintained whilst the glass cools through its strain point, and result in the production of permanent compressive stresses in the surface layers of the glass sheet, with compensating tensile stresses in the centre of the glass thickness. The degree of toughening achieved varies with the rate of chilling and increases with increasing chilling rate.
The quenching jets are usually oscillated multi-directionally over the surfaces of the glass sheet so as to produce a uniformly toughened glass sheet in which the tensile stresses in the centre of the glass thickness are of equal magnitude in all directions of the plane of the glass sheet, by smoothing out the localised cooling effect of the quenching jets impinging on the glass sheet.
Such glass sheets when toughened by air quenching generally have a ratio of surface compressive stress to central tensile stress of about 2:1 and thus the degree of toughening can simply be specified by reference to the central tensile stress present in the glass sheets.
When such uniformly toughened glass sheets are fractured they break up into small particles of glass (termed "dicing").
The manner in which the glass sheets fracture and in particular the fineness of dicing depends on the degree to which the glass sheets are toughened. In general the fineness of the dicing increases with increase in the degree of toughening.
In most countries there are official regulations specifying the fracture requirements for toughened glass sheets which are to be used as side or rear windows for motor vehicles.
Typically such regulations specify that the toughened glass sheet shall be fractured by localised impact at a defined position on the glass sheet, two particular positions being at the geometrical centre of the glass sheet and at a position adjacent the edge of the sheet. It is then required that areas of the fractured glass sheet should be selected where the particle count is a minimum and where the particle count is a maximum and limitations are placed on the minimum and maximum particle counts permissible in such areas. The minimum particle count permissible determines the maximum size of particles resulting from fracture so as to limit the danger of laceration by larger particles subsequent to fracture of the glass sheet in an accident. The maximum particle count permissible determines the minimum fineness of particles resulting from accidental fracture of the glass sheet so as to limit the danger of ingestion of fine glass particles.
At present motor vehicle side and rear windows are made from glass of about 4.0 mm to 6.0 mm thickness and can be uniformly toughened in the manner described above so as to meet official fracture requirements.
For example glass sheets of thickness 4 mm and above meet the proposed E.E.C. standard referred to below if uniformly toughened to have a central tensile stress in the range 55 MN/m.sup.2 to 59 MN/m.sup.2. However, in the interest of reducing weight, there is now a trend towards the use of thinner glass in motor vehicles e.g. of about 3.0 mm thickness, glass of thickness in the range 2.5 mm to 3.5 mm being of particular interest.
In the draft standard under discussion by the European Economic Community (E.E.C) it is required that the number of particles in any 5 cm .times. 5 cm square traced on the fractured glass, excluding a 3 cm wide band around the edge of the glass sheet and a circular area of 7.5 cm radius around the point from which fracture was initiated, should be 50 at the minimum and 300 at the maximum.
It has been found difficult to toughen such thinner glass sheets to meet the official fracture requirements. In particular it has been found that the difficulty of toughening glass sheets of thickness 3.5 mm or less to meet the official fracture requirements, increases with increasing glass size. The difficulty is particularly evident at a size greater than about 1100 mm .times. 500 mm. This is about the size of the smallest vehicle rear window in current production. Many vehicle side windows are also of about this size or greater. Further it has not been found possible to produce uniformly toughened side and rear windows of about 3.0 mm in thickness by conventional air toughening practice, which will meet the requirements of the proposed E.C.C. standard for minimum and maximum particle counts when broken from the specified fracture positions, particularly at the position of the geometrical centre of the glass sheet. If such a glass sheet is uniformly toughened to meet the requirement of the proposed E.E.C standard for minimum particle count it is found that the requirement for maximum particle count is exceeded and vice versa.
The proposed E.E.C. standard also has a requirement that the fractured glass sheet should not contain any elongated particles with jagged ends of more than 6 cm in length, such particles being referred to as "splines". It has also not been found possible to produce a uniformly toughened glass sheet of about 3.0 mm in thickness which will meet this requirement as the fractured glass sheet usually contains splines.
British Standard No. BS 5282 entitled "Road Vehicle Safety Glass" is less restrictive than the proposed E.E.C. standard in that it specifies for glass less than 4 mm in thickness, a minimum particle count of 40 in a 5 cm .times. 5 cm square may be permitted and the maximum permitted particle count in a 5 cm .times. 5 cm square may be 400. The British Standard also basically prohibits the presence of splines of more than 6 cm in length in the fractured test glass. It has been found possible, by close control of existing toughening methods, to produce uniformly toughened glass sheets of about 3.0 mm thickness which will meet the requirements of this British Standard for minimum and maximum particle counts. However such close control is difficult to achieve in routine commercial manufacture.
The main difficulty in complying with the British Standard arises, however, in having to meet the basic requirement that no splines are to be present in the fractured glass sheet.
It has now been discovered that glass sheets of the kind used as motor vehicle side or rear windows, which are from 2.5 mm to 3.5 mm thick, particularly sheets 3 mm thick, can be toughened in a way which meets official fracture requirements such as the proposed E.E.C. standard.